1. Field of the Invention
The present invention relates to a technology of an optical waveguide used in an optical communication.
2. Description of the Related Art
An optical waveguide used in an optical modulator, etc. is formed by using a dielectric substrate (electro-optic crystal) such as a lithium niobate (LN; LiNbO3) substrate and a lithium tantalate (LiTaO2) substrate and by forming and thermally diffusing a film of metal such as titanium (Ti) on a portion of this substrate or performing proton exchange in benzoic acid after patterning. Optical modulation can be performed by disposing electrodes near the optical waveguide to apply a voltage. The light propagating through the waveguide tends to be absorbed by the electrodes, and a buffer layer is disposed between the LN substrate and the electrode to prevent the absorption. silicon oxide (SiO2) with a thickness of 0.2 micrometers (μm) to 1 μm is used for the buffer layer.
To miniaturize the optical modulator, or to form a long optical waveguide within one chip, straight optical waveguides are used in combination with bent optical waveguides. By bending the optical waveguide, the straight optical waveguide can be formed into a bent shape and miniaturized.
FIG. 9A is a plan view of a conventional bent waveguide and FIG. 9B is a cross-section of the conventional bent waveguide. On a substrate (LN substrate) 500 at a bent portion, a groove 505 is formed on the outside of an optical waveguide 502 bent with a curvature radius R to prevent the optical loss due to radiation from the optical waveguide 502 of the bent portion (for example, Japanese Registered Utility Model Publication No. 2537800). The optical waveguide 502 can be formed by diffusing Ti as described above. A reference numeral 503 represents a buffer layer formed on the substrate 500.
FIG. 10A is a plan view of another conventional bent waveguide and FIG. 10B is a cross-section of another conventional bent waveguide. On the substrate 500 at the bent portion, a convex ridge portion 506 is disposed by forming the groove on the inside and outside of the optical waveguide 502, and the optical waveguide 502 is formed in the ridge portion 506. Such ridge portion 506 enhances confinement of the light within the optical waveguide 502, thereby reducing the optical loss at the bent portion.
In some cases, the buffer layer 503 is formed with a material having a refractive index at the same level as or higher than that of the substrate 500. By increasing an amount of light leaking into the buffer layer 503, the confinement can be enhanced. Other materials of the buffer layer 503 include titanium oxide (TiO2), etc. The refractive index of TiO2 is approximately 2.3, which is higher than that of the LN substrate 500. With the optical waveguide using TiO2, a mode size, i.e., a waveguide mode size of fundamental wave propagating through the optical waveguide can be adjusted to improve efficiency of the propagation of the light in the optical waveguide (for example, Japanese Patent Laid-Open Publication Nos. H7-20508, H7-199238, and 2003-29307).
However, an optical waveguide formed by diffusing Ti does not confine light sufficiently, therefore a large light loss occurs. If Ti is diffused from the surface of the substrate, the density thereof becomes the highest at the surface of the substrate and is gradually lowered as the depth of the substrate increases. Although a buffer layer of SiO2 etc. is disposed on the surface, SiO2 has a low refractive index, which is about 1.5 and less light leaks into the buffer layer. Therefore, the center of the light is away from the substrate surface, which has a high refractive index, resulting in poor confinement.
Therefore, if the buffer layer contacting the substrate is formed by using TiO2 etc., which have a refractive index at the same level as or higher than that of the substrate, the light amount leaking into the buffer layer can be increased to enhance the confinement of the light. However, if this configuration is directly applied to the optical waveguide with grooves arranged along the sides, since a refractive index is increased on the side and bottom surfaces of the grooves, the light is confined poorly in the lateral direction. Therefore, the light loss increases.
In the case of an optical modulator, etc., which modulate light with electrodes disposed on the optical waveguide, if the buffer layer is made only of a film with a high refractive index such as TiO2, a mode of the light extends over the electrodes and is absorbed by the electrodes, and the light loss increases.